Transcript Subtle Imipenem Resistance In an ICU

Subtle Imipenem Resistance In an ICU Outbreak of
Acinetobacter-baumanii calcoaceticus (ACBC)
Sandy J. Close, Pharm.D., BCPS, Steven J. Martin, Pharm.D., BCPS, FCCM,
Eric Sahloff, Pharm.D., Diane Cappelletty, Pharm.D
The Infectious Disease Research Laboratory; College of Pharmacy, The University of Toledo
2801 W. Bancroft St., Toledo, Ohio 43606
ABSTRACT
Hypothesis/Methods: 25 consecutive isolates from nosocomial
infections were collected, and baseline broth-dilution minimum
inhibitory concentrations (MIC’s) for imipenem were determined.
Isolates (inoculum 108 cfu/ml) were then plated on Mueller-Hinton
agar (MHA) containing 2X and 4X the baseline MIC. Growth on
antibiotic plates was evaluated at 24 and 48 hours for the presence
of resistant subpopulations in the inoculum.
Results: Baseline imipenem MICs ranged from 0.25 to 1 g/ml. At 24
hours, 9 (36%) isolates had growth on 2X plates (mean 9.17 x 104
cfu/ml, and 3 isolates on 4X plates (mean 1.63 x 102 cfu/ml). All
growth at 24 hours had MICs 2-4X baseline. At 48 hours, 18 (72%)
isolates had growth on 4X plates (mean 4.0 x105 cfu/ml). For 48 hour
growth, 8 isolates had MICs >2X baseline, and 10 had no change in
MIC. Isolates with rapid reversion to baseline MICs likely represent
either unstable resistance, or drug degradation in agar at 37C.
Figure 2. ACBC
Isolate 10
100
80
8
M10
7
60
Log cfu/ml
Number of Isolates
Introduction: Multi-drug resistant isolates of ACBC were collected
from an outbreak in our 20 bed medical/surgical ICU. Imipenem
monotherapy has been the treatment of choice for these ACBC
infections. In vitro modeling uncovered the presence of subtle,
underlying imipenem-resistant subpopulations in 4 antibiotic-naive
ACBC isolates from this outbreak. The purpose of the present study
was to determine the extent of imipenem-resistant subpopulations in
ACBC isolates from our ICU to more accurately predict the likelihood
of successful monotherapy with imipenem.
120
Figure 1.
ACBC Incidence 1998-2002
40
20
I10
6
5
4
3
0
1998
1999
2000
2001
2002
Year
METHODS
25 consecutive isolates from nosocomial infections were collected,
and baseline broth-dilution minimum inhibitory concentrations
(MICs) for imipenem were determined. The isolates (inoculum 108
cfu/ml) were then grown on Mueller-Hinton agar (MHA) containing
2X and 4X the baseline MIC. Growth on antibiotic plates was
evaluated at 24 and 48 hours for the presence of resistant
subpopulations in the inoculum.
For isolates with subpopulation growth on the antibiotic plates at 48
hours, MIC’s were re-determined, for comparison with baseline. In
an additional project, four isolates from this sample were selected
for additional testing in an in-vitro pharmacodynamic model. Pulse
field gel electrophoresis studies are in currently in progress.
2
1
0
2
4
6
8 10 1224
30
36
42
48
Time (hrs)
M = Meropenem at 1 gram q 8 hours
I = Imipenem at 500 mg q 6 hours
In vitro modeling confirmed the presence of
underlying imipenem-resistant subpopulations in
4 antibiotic-naive ACBC isolates from this
outbreak. An example of resistance selection is
shown For more detail refer to Poster # 617.
RESULTS
Conclusion: Nearly 75% of ACBC outbreak isolates demonstrated
subtle but significant elevations in imipenem MICs at baseline,
which are not evident in routine MIC testing. Resistance was
evident after brief exposure to low drug concentrations. An
unstable resistance pattern was observed with higher drug
concentration exposure. The presence of significant imipenemresistant subpopulations in ACBC raises the concern for imipenem
monotherapy in the management of these infections.
Baseline
MIC (mcg/ml) range 0.125-1
CONCLUSIONS
24 hours
48 hours
0.25-2
0.125-1
MIC90 (mcg/ml)
0.25
2
0.5
Growth: 2X MIC
--
9/25 (36%)
24/25 (96%)
Growth 4X MIC
--
3/25 (12%)
18/25 (72%)
48 hour growth:
•8 isolates had MICs  2X baseline
BACKGROUND
Acinetobacter strains have become significant nosocomial
pathogens in recent years. Intensive cares units, burn units and
extended care facilities are the most common locations of these
outbreaks.1,2,3 S Susceptibilities to antimicrobial agents vary
greatly from institution to institution and can rapidly change
during an outbreak.
•10 isolates had no change in MIC from baseline
Preliminary Pulse-Field Gel
1
2
3
4
5
•Over one half of the ACBC outbreak isolates
demonstrated subtle but significant imipenem
resistance at baseline, which is not evident in
routine MIC testing.
•Resistance was evident after brief exposure to low
drug concentrations (similar to clinical conditions).
• An unstable resistance pattern was observed with
higher drug concentration exposure.
•The subtle presence of significant imipenemresistant subpopulations during an ICU ACBC
outbreak raises the concern for imipenem
monotherapy in the management of these
infections.
•Further study including combination therapies is
warranted. Pulse-field gel analysis is currently
being undertaken to further examine these isolates.
REFERENCES
The incidence of ACBC infections at our institution has been on
the rise during the last two years (Figure 1). Simultaneously
susceptibilities to commonly prescribed antimicrobial agents have
been decreasing. Traditionally, the drug or drug regimen of choice
for the treatment of these infections has been determined by
evaluating the MIC data provided by the microbiology laboratory.
1BergogneBérézin
E. The increasing significance of
outbreaks of Acinetobacter spp.: the need for control and
new agents. J Hosp Infect 1996;30(suppl):441-52.
2Jellison
TK, McKinnon PS, Rybak MJ. Epidemiology,
Resistance, and Outcomes of Acinetobacter baumanii
Bacteremia Treated with Imipenem-Cilastatin or
Ampicillin-Sulbactam. Pharmacotherapy 2001;21:142-48.
Imipenem monotherapy has been the treatment of choice for these
ACBC infections. The purpose of the present study was to
determine the extent of imipenem-resistant subpopulations in
ACBC isolates from our ICU to more accurately predict the
likelihood of successful monotherapy with imipenem.
3
•Lanes
1-3 = Different ACBC Strains
•Lane 4 = Insufficient DNA to Assess
•Lane 5 = Control
Koeleman JGM, Parlevliet GA, Dijkshoorn L, Saveldoul
PHM, Vandenbroucke-Grauls CMJE. Nosocomial
outbreak of multi-resistant Acinetobacter baumanii on a
surgical ward: epidemiology and risk factors for
acquisition. J Hosp Infect 1997;37:113-123.